In wireless communication systems, a plurality of wireless communication devices (WCDs) communicate with one or more base stations within areas known as cells. A wireless communication system can include a variety of types of WCDs, including, for example, wireless telephones and devices having wireless communication capabilities, such as personal digital assistants (PDAs) and modems for use with laptop computers. Wireless communication functions can also be incorporated into other types of devices, such as automobiles. With wireless capabilities incorporated into an automobile design, a driver can obtain real-time, location-based traffic, weather, and navigation information, as well as roadside assistance and vehicle condition alerts.
Within each cell, several WCDs may communicate with a base station simultaneously using a single frequency band. Sharing of the frequency band can be accomplished using any of a variety of multiple access techniques. One technology that has enjoyed rapid growth is code division multiple access (CDMA). In CDMA systems, speech or data is converted to a digital form, which is then transmitted as a radio signal. Each call is distinguished by a unique code. In particular, each WCD uses a unique spreading code to modulate the signals it transmits and to demodulate the signals it receives. This code is added to the information data, e.g., the voice data, and modulated onto the carrier. An identical code is used in the receiver that is used to correlate the code with the carrier. The correlation process passes only data that matches the code. Thus, non-valid signals, e.g., signals from other users, are not decoded and appear as noise. As a result, minimal interference between WCDs is achieved. Accordingly, several WCDs can share a single frequency band. Further information regarding CDMA systems is set forth in the well-known IS-95 standard.
Some other wireless communication systems use other multiple access technologies, including, for example, frequency division multiple access (FDMA), amplitude companded single sideband (ACSSB) and other amplitude modulation (AM) schemes. Another technology that has come into widespread use is time division multiple access (TDMA), in which WCDs communicate during allocated time slots.
One multiple access technology related to TDMA is known as Global System for Mobile (GSM), which uses TDMA in combination with encryption techniques. GSM communications are structured using a number of channels, including, for example, a traffic channel (TCH) for transferring information. In addition, the GSM standard uses a common control channel (CCCH) for transferring control information, such as WCD registration, paging, and call origination and termination. The CCCH itself includes several channels associated with specific types of control information. These channels include the random access control channel (RACH), the paging and access grant channel (PAGCH), the broadcast control channel (BCCH), the synchronization channel (SCH), and the frequency correction channel (FCCH). The FCCH and SCH, for example, are used in maintaining synchronization between WCDs and respective base stations.
GSM channels are communicated using any of a number of frequencies within an allocated band of frequencies, for example, 880–960 MHz. In addition, DCS channels can be communicated using a different band of frequencies, including, but not limited to, the 1710–1880 MHz band. The WCD generates these frequencies using a clock arrangement. The clock arrangement may incorporate an oscillator, such as a voltage-controlled crystal oscillator (VCXO) or a voltage-controlled, temperature-compensated crystal oscillator (VCTCXO). In some implementations, the oscillator is controlled by a pulse density modulator (PDM). Among other operational aspects, the PDM controls the frequency generated by the oscillator. In order for the WCD to communicate with its associated base station, it is desirable that the oscillator generate a frequency that matches the frequency on which the base station is communicating.
To ensure that the WCD and base station frequencies match, the clock arrangement in the WCD may use automatic frequency control (AFC) techniques to track the frequency used by the base station and match that frequency. AFC techniques generally involve estimating a frequency error between the WCD and base station frequencies, i.e., the difference between the WCD frequency and the base station frequency. The estimated frequency error is used to determine a frequency offset to be applied to the oscillator to compensate for the estimated frequency error. The PDM then applies the frequency offset to the oscillator, for example, by applying an input voltage to the oscillator, which alters the oscillator frequency in response to the applied voltage.
The WCD can implement AFC using any of a number of techniques. One conventional AFC technique involves the use of an infinite impulse response (IIR) filter to estimate the frequency error. The IIR compares a long-term frequency error estimate with an instantaneous frequency error estimate for an individual sample to determine the difference between the long-term and instantaneous frequency error estimates. The IIR adjusts the long-term frequency error estimate as a function of this difference, typically by multiplying the difference by a scale factor and using the resulting product to adjust the long-term frequency error estimate. Using a large scale factor enables the IIR to respond relatively quickly to changes in the frequency error, but also renders the WCD susceptible to noise, e.g., variations in the instantaneous frequency error estimate from one sample to the next. Such variations may or may not be caused by a change in the actual frequency error. Alternatively, the IIR can use a small scale factor to reduce susceptibility to noise. Using a small scale factor, however, increases the response time of the IIR to changes in the actual frequency error.
Another conventional AFC technique involves averaging frequency error estimates over a predetermined number of samples, or integration length, to determine an average frequency error estimate. If the integration length is small, the WCD can respond quickly to changes in the frequency error. With a small integration length, however, the AFC technique is susceptible to noise. Using a large integration length reduces the susceptibility of the AFC technique to noise. As a result, the steady state response to a change in the frequency error is more stable relative to smaller integration lengths. However, the WCD responds to changes in the frequency error more slowly relative to smaller integration lengths.